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I'm going through Jeffrey D. Ullman's Introduction to Automata Theory, Languages, and Computations. The author reduces an instance of the membership problem in $L_d$ (diagonalization language) to a membership problem in $L_u$. Those are defined as follow:

$L_d = \{\langle M\rangle \mid \langle M\rangle \notin L(M)\}$

$L_u = \{\langle M, u\rangle \mid u \in L(M)\}$

We start off with assuming $L_u$ is recursive, reduce the problem in $L_d$ to a problem in $L_u$, and solve $L_d$ using the assumption that $L_u$ is recursive, now since $L_d$ is not even recursively enumerable, our assumption that $L_u$ is recursive turns out to be wrong, and since $L_u$ has a TM which accepts it, we conclude $L_u$ is a RE but not recursive language. A few pages later, the books says:

Theorem 9.7: If there is a reduction from $P_1$ to $P_2$ then

a) If $P_1$ is undecidable then so is $P_2$

b) If $P_1$ is non RE then so is $P_2$

I'm confused about the part b) of this theorem, it says reducing a non RE problem to another problem gives you a non RE problem, but having reduced an instance of $L_d$ to an instance of $L_u$, we have reduced a non RE problem to a RE but not recursive problem.

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If you look closely at the proof to show that $L_u$ is not recursive, the autor is using a reduction to $\overline{L_u}$ (which is not RE), not to $L_u$, and explain that a reduction to $L_u$ cannot work since $L_u$ is RE.

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  • $\begingroup$ Thank you for your answer and for helping with the formatting. My professor reduced the problem to $L_u$, I cannot see the problem with doing that, given that we are starting off with the assumption $L_u$ is recursive, it does not matter if we reduce to the complement or to $L_u$ itself, since both would be recursive, the only change we would have is, in the diagram Ullman has constructed, we would have hypothetical algorithm M for $L_u$, and if the hypothetical algorithm accepts, our machine for $L_d$ will reject, and vice-versa. $\endgroup$
    – Yueor
    May 19 '21 at 9:34

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